1. Field of the Invention
The present invention relates to hydraulic actuators of the type used to exert large forces through a rod when hydraulic pressure is applied to the actuator, which may be a hydraulic cylinder for reciprocatably driving the rod or a rotary actuator for rotatably driving the rod. More specifically, the invention is a rod locking device for locking the rod of a hydraulic actuator in a desired position.
2. Description of the Prior Art
It is highly desirable to be able to lock the driven rod in a hydraulic actuator for safety reasons in case of a sudden loss of pressure as well as to avoid small, but undesirable movements of the rod due to leaking of hydraulic fluid past the seals within the actuator or because of compressibility of the hydraulic fluid. Conventional locking devices are generally either the mechanical type or the hydraulic type. Mechanical types hold the rod in the desired position better, but the hydraulic types are simpler. The hydraulic types work by blocking the column of hydraulic fluid and are the least expensive and usually are smaller than the mechanical types.
In some applications, mechanical locking is the only acceptible solution. For example, locking must be done mechanically in a machine station on a production line where the hydraulic actuator muct maintain a very precise position for a reasonable length of time to produce an acceptible product. In contrast, a hydraulic column blocking type locking device is all that is required in the actuating cylinders on the scoop or blade of a bulldozer, where a high degree of blade position is not required and the duration in the locked position is usually very short.
Mechanical locking devices can be subdivided into three groups, particularly for hydraulic cylinders. The first is end position locking, which locks at stroke extremes only and is best illustrated by those using a toggle or ball mechanism which is spring-loaded-to-engage and pressure-to-release device providing a mechanical connection between the piston and cylinder barrel or end cap.
The second type of mechanical lock device is the incremental locking type, which locks in specific stroke positions (for example, every 1/16 of an inch of stroke). An example is a unit using a helical groove on the piston rod which engages a gear that is locked by a panel-type mechanism. Locking can only be incremental because the panel must engage discrete tooth positions on the gear.
The third type of mechanical locking device is the infinite position type which locks at any point of stroke. This type has the broadest application possibilities. One form is a hydraulic cylinder with an integral Acme screw. External power is needed to move the screw with the piston. Another form is a central ball screw which follows the hydraulic piston through its stroke. The locking function is provided externally by applying a brake to the screw.
These types of mechanical locking devices for hydraulic cylinders are used in many industrial applications where they provide the precision and durability advantage of the mechanical lock. However, they do involve rather complex and expensive mechanisms to attain a locking action, particularly the infinite position type. The first two mentioned mechanical locking types are generally less complex, but they do not provide the ability to lock the piston rod in any position.
One device which combines the positive holding advantage of a mechanical lock with infinite position lock capability of a blocked fluid column is the Bear-Loc device, which consists of a rod, a barrel and a locking sleeve. The sleeve is an extension of the cylinder rod end cap, which forms an interference fit with the outside diameter of the rod. When hydraulic fluid under pressure is introduced between the sleeve and rod, the sleeve expands radially, providing enough clearance to allow the rod to rotate or stroke. The lock re-engages automatically when fluid pressure is removed. However, in spite of its advantages the Bear-Loc device is complex, difficult to install and remove, and not readily adaptable to different size rods.